Pottery: Experiments with Glazes

Iron glazes have their own page.

Willett Cobalt Green:
Gerstley borate 49g
silica 32g
EPK 19g
Bentonite 2g
chrome oxide 2g
cobalt carbonate 1.5g
COE: 5.2x10-6/K
Stoneware (Tucker Smooth White)
Source: Jim Willett
Painted thickly on bisque, fired cone 6 electric. This glaze needs to be applied very thickly to keep its colour, but doesn't sag or move. Expansion is a bit low, so avoid high-expansion clays. Vase is 10 cm high.

Boron Blue:
Fusion F12 frit 33g
Fusion F2 frit 26g
silica 24g
EPK 17g
copper carbonate 4g
cobalt carbonate 1g
white glue 0.3g
COE: 5.5x10-6/K
Stoneware (Tucker Smooth White), built from rolled slab.
Add the white glue after sieving to keep the colorants in suspension. Painted on bisque medium thick. Fired cone 6 electric, held at maximum temperature (1210°C) for 30 min, kiln off. Drip catching bowl was needed, and grinding of base after. High gloss over all areas, no blisters, some of the colour phases but none of the crystallization that some have obtained. Has to be thick to get variations in colour. Probably best on work that has horizontal surfaces to contrast with vertical. Vase is 10 cm tall.

Selsor Waxy White:
nepheline syenite 30g
Gerstley borate 20g
talc 18g
zirconia 15g
EPK 13g
silica 10g
calcite 9g
COE: 5.6x10-6/K
Stoneware (Tucker Smooth White), finger shaped.
Source: Marica Selsor
Painted thick on bisque. Fired cone 6 electric, held at maximum temperature (1220°C) for 10 min, kiln off. A soft white shading to light brown where thin. Even on a child's first finger pot, it gives a smooth surface with a lovely feel and excellent coverage. Mug is 6 cm tall.

Turnidge Crystalline:
Fusion F75 frit 50g
zinc oxide 22.5g
silica 22.5g
lithium carbonate 5g
manganese dioxide 3g
cobalt carbonate 1g
COE: 7.7x10-6/K
Stoneware (Tucker Smooth White)
Painted thin on bisque, fired cone 6 electric. Crystalline glazes are known to be tricky. For me, crystals formed only at the edges of the blue with short soaks (30 min at 1000°C here); longer soaks resulted in the blue taking over completely. I couldn't persuade it to form the circular crystals that some have obtained. Its expansion is too high for it to be a functional glaze. Bowl is 8 cm diameter.

Zakin Clear
nepheline syenite 51g
Gerstley borate 22g
Wollastonite 14g
EPK 8g
zinc oxide 3g
titanium dioxide 2g
COE: 7.3x10-6/K
Stoneware (Tucker Smooth White)
This is a fluid glaze that can be applied very thin and still seal evenly. Excellent for applying over non-toxic underglaze, but has too little silica to be functional with any colorant that could cause problems if it leached. It's expansion is high, but it withstands the usual freezer to boiling water test when dipped thinly on my stoneware. It will be safest on high-expansion clays.

Marilee Lava:
Custer feldspar 50g
calcite 24g
EPK 13g
silica 13g
titanium dioxide 11g
Mason 6600 stain 10g
silicon carbide 600 mesh 0.3g
COE: 6.7x10-6/K
Stoneware (Tucker Smooth White), built from rolled slab.
Dipped on bisque, fired cone 6 electric. This glaze gets its texture from calcite decomposition at 870°C (it's 40% CO2) followed by silicon carbide decomposition when alkalis in the glaze melt releasing more CO2 bubbles, and by being underfired therefore stiff at cone 6. Without the stain, it's white and can probably accept most colorants. More suitable for large scale pieces. Vase is 12 cm high.

Spectrum "Satin Clear" glaze:
My first experiments - no photos taken. Applied as recommended in a thin layer with a brush, this glaze turned out to not be clear at all! It looked just like skim milk, even where brushed so thin it didn't quite seal the surface. My teacher fired it again in her kiln, at cone 8, in case the milkiness was due to underfiring. It wasn't. And, when my supplier phoned Spectrum, she was told airily that "all matte glazes are milky"! (Needless to say, her reaction and mine was, "Then why do you label it clear?") Scrub one vase and one manufacturer.

Experiments with other clear gloss overglazes showed that some are not compatible with Spectrum underglazes - the underglazes bleed through the overglaze. One overglaze even destroyed the red colour. And, neither glaziers' etching paste (acid) or concentrated Drano (alkali) had any effect on the gloss of any gloss overglaze I tried.

Cooling rates:
My kiln is a third-hand Olympic 1818WH electric manual single-brick-lined unit 43 cm diameter and 43 cm high inside. Switched on lowest power, it takes 1 hr to reach 500°C then on high power a further 2 hr to reach cone 6, 1220°C. After a 10 min hold at 1220°C it takes 30 min when switched off to cool to 950°C, my usual hold temperature for iron glazes.

John Sankey
other notes on pottery